1. Field of the Invention
The present invention relates to a transparent ultraviolet curable (abbreviated as UV-curable) adhesive, and more particularly to a transparent UV-curable adhesive composition is provided with high transparency, high refractive index, excellent adhesion, and excellent workability, and is so suitably applied to be adhered to displays, a transparent conductive film and touch panels.
2. Description of Related Art
Optical transparent adhesive is used in components for touch devices to bind the cover lens and ITO conductive glass or thin film together, and is also used to bind a touch module and an LCD display module together. Such adhesive must have high photopermeability and proper refractive index in order to maintain good visual effects of the resulting products. When used to bind a touch module and an LCD display module together, an adhesive layer has to compensate thickness difference caused by printing, and thus its thickness is usually of 175 μm or more for effective height compensation among various printed patterns and wiring. For the applied adhesive layer to meet the industrial requirements, it has to be thermally stabile, durable, bubbleless, not coming off, and crackless and show no apparent defects.
UV-curable adhesive is free of solvent and thus eliminates the need of a solvent-removing step when forming an adhesive layer. As compared to solvent-containing adhesive, UV-curable adhesive tends to have a thickness more than 175 μm and is unlikely to generate volatile organic compounds (VOCs). For its environmental and other advantages, UV-curable adhesive has been extensively used in manufacturing of optical products.
Light reflects, refracts and diffuses differently in media having different refractive indexes. Such reflection, refraction, and diffusion may have adverse effects on light extraction of products using these media. This problem can be prevented by choosing a materiel of the adhesive layer that has a refractive index similar to that of the substrate so as to enhance transmittance.
Generally, substrate for optical devices is glass. Glass has a refractive index of 1.52. However, currently in optical devices the adhesive layer is usually made of acrylate. Acrylic adhesive may have a refractive index between 1.46 and 1.47, which is different from the refractive index of glass and can degrade light extraction of the resulting optical device. U.S. Pat. No. 7,911,564 discloses a photocurable composition containing polyurethane acrylate (or polyurethane methyl acrylate) and a photocurable prepolymer. However, the composition is poor in photopermeability. For improving transmittance of displays, it is necessary to use optical adhesive that is close to glass in terms of refractive index.